Steam generators of pressurized water nuclear reactors include tubes folded into a U, having two branches clamped at their ends in a tube plate. The tubes are arranged in adjacent planar layers. The curved parts of the tubes of the bundle which are juxtaposed at its upper part have radii of curvature which are different from each other and are placed adjacent to constitute a structure of substantially hemispherical shape termed the tube bend region.
During the operation of the steam generator, high-temperature pressurized water flows in the tubes of the bundle and feedwater is brought into contact with the outer surface of these tubes, along which it moves in the vertical direction while heating and then vaporizing, to reemerge in the form of steam at the upper part of the generator.
The flow of the fluid in contact with the tubes may cause vibrations which can lead to damage of tubes if they are not held efficiently.
The straight part of the tubes is engaged in spacers situated at regular distances from each other along the height of the bundle. These straight parts are therefore effectively held by rigid elements. The curved parts of the tubes of the bundle which constitute the tube bend region must also be held, and for this purpose spacer bars are interposed between two adjacent tube layers of the bundle and are arranged along substantially radial directions of the tube bend region.
These spacer bars are generally connected in pairs in an articulated manner at the one of their ends which is arranged inside the tube bend region and are placed angularly to constitute V-shaped structures.
The outer ends of the antivibration bars which are opposite their central part project with respect to the tubes constituting the outer level of the tube bend region and are connected together by linkage means which hold the antivibration bars.
Various means for linking of the outer ends of the antivibration bars have been proposed, using fastening elements placed above the upper surface of the tube bend region.
It has, for example, been proposed to weld the ends of the antivibration bars onto curved elements arranged along meridian planes of the tube bend region.
It has also been proposed to fasten the ends of the antivibration bars, for example by welding, onto substantially parallel rings or retaining pins arranged above the upper surface of the tube bend region.
In any case, the fastening of the antivibration bars requires complex operations most often involving welding which must be carried out in the vicinity of the tubes of the bundle, which thus risk undergoing some degree of damage. Furthermore, the materials constituting the antivibration bars must be chosen to allow their welding and optionally their treatment after welding, under good metallurgical conditions.
Mechanical linkage devices have also been proposed which make it possible to connect the outer ends of the antivibration bars to fastening components such as retaining rings arranged above the outer surface of the tube bend region.
These devices are relatively complex and require for their fastening the use of heavy and bulky additional elements attached onto the tube bend region. Furthermore, these devices do not ensure very close alignment of the antivibration bars and precise adjustment of the clearance between two successive antivibration bars between which a layer of tubes is interposed.
In FR-A-2,664,965, shows a blocking device which includes means of linkage between the outer ends of antivibration bars arranged in sets of at least two bars, in which the bars are aligned and include aligned through openings, one of which is tapped. The means of linkage of the antivibration bars include a pin which is introduced into the aligned openings of the set of antivibration bars and into the bore of spacers which are each interposed between two successive antivibration bars. The pin, which has a threaded end, is screwed in the tapped opening of an antivibration bar situated at the end of the row. The pin is also solidly attached to a rotational locking element which can be welded onto a spacer. Such a device has advantages over prior art mechanical devices, but requires the use of pins whose length is adapted to the number of bars and to the length of the row of antivibration bars whose ends are assembled together. In certain cases, it is necessary to use a very long pin, which may present problems as regards the conditions of placement and the mechanical strength of the pin.